1. Field of the Invention
The present invention relates to an image pickup apparatus and a control method therefor, as well as a computer-readable storage medium storing a control program for implementing the method, and in particular to a method of displaying a shooting azimuth in an image pickup apparatus having an electronic compass function.
2. Description of the Related Art
In recent years, there has been a growing need in a digital camera for knowing a shooting position and a shooting direction, and those which have a GPS system and an electronic compass function incorporated therein or those which work in cooperation with external GPS equipment have come onto the market.
A main use of an electronic compass in a digital camera is to detect an azimuth of a central part of a taken image (in the direction of the center of a lens' optical axis) and record the azimuth as shooting azimuth data in association with taken image data. On the other hand, an electronic compass function of a digital camera can assist shooting by displaying a shooting azimuth in real time on a display and an EVF on a rear side of the digital camera during shooting.
There has also been proposed a method that, when displaying a shooting azimuth, expressing the shooting azimuth using a figure or a scale because the shooting azimuth displayed merely in numerical form is not easy to understand. For example, Japanese Laid-Open Patent Publication (Kokai) No. H11-155124 discloses a technique according to which environmental conditions during shooting such as position, azimuth, and temperature are defined as graphics and displayed in combination with a taken image. Also, Japanese Laid-Open Patent Publication (Kokai) No. 2008-17223 discloses a technique according to which a guide image indicative of a placement of a subject is created from information on an installation location of an apparatus, information on a pose of the apparatus, and time information, and the created guide image is displayed in a manner being superimposed on a live view image. For example, when the night sky is to be shot, an image of a constellation is displayed as a guide image, enabling a user to shoot a desired constellation.
Expressing a shooting azimuth using a figure or a scale or creating and displaying a guide image as described above enables a user to visually recognize a shooting azimuth and use the same for framing during shooting. Further, using positional information, pose information, time information, information on an angle of view, and so on in combination makes it possible to estimate the position and size of a subject itself, which can be used as very useful assist information during shooting.
However, in the case of displaying a though-the-lens image from an image pickup device in real time on a display or an EVF mounted on a rear side of a digital camera for the purpose of framing during shooting, it is necessary to update a though-the-lens image at a frame rate of 15 to 30 frames per second. At the same time, in the case of displaying an azimuth using a figure and a scale, the amount of computations per unit time is large, and hence it is difficult for an arithmetic device with low processing power such as one mounted on a digital camera to produce complicated displays.
Moreover, because the angle of view in shooting does not change when a taking lens is a single focus lens, the interval between ticks does not have to be variable. On the other hand, the angle of view in shooting changes when a taking lens is a zoom lens, the interval between ticks has to be adjusted in real time according to zoom position so that a view of a through-the-lens image being displayed and a view of a shooting azimuth around a perimeter of a screen can be coincident with each other. Such real-time operations require an enormous amount of computations, and it would be much more difficult for an arithmetic device with low processing power to realize this. Even if this can be realized, other processes important for shooting such as a process in which a face is detected, a process in which a subject is followed, and a process in which autofocus is continuously performed (continuous AF) may be affected. The same holds for not only displaying of a through-the-lens image during taking of a still image but also displaying of a through-the-lens image during taking of a moving image.